Plant for the controlled-speed pneumatic transport of granular material and conveyance speed control process

ABSTRACT

The invention relates to a plant for the transport of granular material comprising at least one container for granular material to be transported, at least one receiver-meter group, at least one conveyance duct, depressurization-pressurization means arranged to suction/inject a gaseous medium from/into said at least one container, and at least one vacuum duct between a receiver-meter group and said depressurization-pressurization means, thereby creating a flow of said granular material and said gaseous medium in said at least one conveyance duct and a flow of gaseous medium between said at least one receiver-meter group and said depressurization-pressurization means.

FIELD OF INVENTION

The present invention regards a plant for the controlled-speed pneumatictransport of granular material, particularly but not exclusivelysuitable for the transport of granular material made of plasticmaterial, as well as a process related thereto.

BACKGROUND OF INVENTION

With the terms “granules” or “granular”, it is intended to indicate inthe present description and in the claims the small scales, sheets orplates produced by the grinding-crushing of slab, sheet, film and thelike plastic material. In the plants for working and/or transformingplastic materials reduced into granules, granular material istransported from a storage container to one or more machines designed touse such a material and usually comprising injection or thermoformingpresses, by means of a pneumatic conveyance or transport system,preferably operating under reduced pressure. The transport system mustensure a minimum flow rate of granular material, thereby ensuring acontinuous feed of granular material to the transformation machine ormachines.

In reduced-pressure transportation systems of granular material proposedup to now, a vacuum source is provided, e.g. a vacuum pump, arranged tosuck air from a container of granular plastic material. The granularmaterial is thus driven by the suctioned air along a suction tubingwhich leads above, and discharges the granular material into, acollection tank, whereas the transport air is suctioned to conveytowards the vacuum source. Between the collection tank of the granularmaterial and the vacuum source, a filter is provided to filter the air,which has just separated from the bulk of the granular material, beforeit reaches the vacuum source. An electronic control unit controls theentire cycle. As a matter of fact, it is the atmospheric pressure thatpushes the granular material along the tubing towards the vacuum source.

For a correct conveyance of the granular plastic material within theducts or tubing, the air flow created by the vacuum source must flowwithin a desired speed range, both to prevent the material from beingconveyed at overly high speeds deemed “dangerous”, and to prevent thestagnation of the granular material if the conveyance speed is notsufficiently high.

One of the most difficult problems to solve in the reduced pressuretransport of granular material within conveyance ducts is that of beingsuitable for maintaining its transfer speed constant, even with thechange of light or section of the ducts and/or configuration (curved,rectilinear) of the tubes along which the conveyance is carried out.

In the conventional plants, and in particular along the transporttubing, the speed of a granular material is usually not maintainedconstant over time. In the various conveyance steps carried out in aconventional reduced-pressure transport plant, the conveyed plasticmaterial granules usually reach very high speeds, even double theoptimum speed. When high speeds are reached, plastic material granulesscrape against the walls, especially at the curved tubing sections, anddue to the combined effect both of the centrifugal force and theelectrostatic charges and to the friction they tend to adhere to thewalls and to form thin film encrustations or deposits on the wallsthemselves. Such deposits, after a certain lapse of plant functioningtime, are detached from the tubing walls, giving rise to multilayercrusts or scales of materials that are even different from each other,considering that they are usually fed in different cycles through onesame tubing. The multilayer crusts or scales that are detached from thewalls constitute a source of pollution/contamination for the granularmaterials that are conveyed along the tubing after their detachment fromthe inner wall of the tubing itself. This phenomenon is called “angelhair” formation in jargon.

SUMMARY OF THE INVENTION

The main object, therefore, of the present invention is that ofproviding a plant for the reduced pressure transport of granularmaterial along tubing in optimal flow speed or intensity conditions forthe specific transported granular material, thus avoiding both theformation of granular material on the walls of the tubing and undesiredstagnations of the granular material.

Another object of the present invention is to provide a plant for thereduced pressure transport of granular material that permitssignificantly reducing the operating costs with respect to theconventional plants.

Another object of the present invention is that of providing a processfor transporting granular material that provide for adapting the flowspeed or intensity to the specific granular material to be conveyedalong the transport ducts.

According to a first aspect of the present invention, a plant for thetransport of granular material comprising at least one container for atleast one granular material to be transported, at least onereceiver-meter group designed to receive granular material from said atleast one container, at least one conveyance duct of said granularmaterial from said at least one container to said at least onereceiver-meter group, depressurization-pressurization means arranged tosuction/inject a gaseous medium from/into said at least one container,and at least one vacuum duct between said at least one receiver-metergroup and said depressurization-pressurization means, thereby creating aflow of said granular material and said gaseous medium in said at leastone conveyance duct or line directed to said at least one receiver-metergroup and a flow of gaseous medium between said at least onereceiver-meter group and said depressurization-pressurization means,said plant comprising detection means of parameters of said flow locatedin said at least one vacuum duct or line, adjusting means of the powerof said depressurization/pressurization means and electronic controlmeans designed to receive in input control signals from said speeddetection means and to emit control signals in output for driving saidadjusting means.

According to another aspect of the present invention, a conveyance speedcontrol process is provided of a granular material along at least oneconveyance line between at least one container of the granular materialto be conveyed and at least one receiver-meter group of the samecomprising:

-   -   the application of a depressurization-pressurization to said        granular material through at least one conveyance duct extending        between said at least one container and said at least one        receiver-meter group and through said at least one vacuum line        or duct, whereby suctioning/injecting a gaseous medium from/into        said at least one container and to create a flow of said        granular material and said gaseous medium along said at least        one conveyance duct directed to said at least one receiver-meter        group and a flow of gaseous medium between said at least one        receiver-meter group and depressurization-pressurization means,        comprising:    -   detecting parameters of said gaseous flow in said at least one        vacuum line or duct, and    -   adjusting said flow by varying the depressurizing-pressurizing        power of said depressurization-pressurization means as a        function of the detected parameters of said flow.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects and advantages of the present invention will be clearerfrom the following detailed description of several currently preferredembodiments of a reduced-pressure granular material transport plant,illustrated as indicative and non-limiting examples in the accompanyingdrawings, in which:

FIG. 1 is a schematic front elevation view of a conventional reducedpressure transport plant;

FIG. 2 illustrates an enlarged scale detail of the plant of FIG. 1 in afirst operating position;

FIG. 3 shows the detail of FIG. 2 in a second operating position;

FIG. 4 is a diagrammatic view of a centralized reduced-pressuretransport plant of granular material from several granular materialsources and the same number of transforming machines of the samematerial;

FIG. 5 shows a partial, schematic view on an enlarged scale of acleaning device of the granular material conveyance ducts provided inthe plant of FIG. 4;

FIG. 6 illustrates a centralized reduced pressure transport plant ofgranular material from several granular material sources and the samenumber of transforming machines, the plant being provided with a set ofcleaning devices as illustrated FIG. 5; and

FIG. 7 illustrates a schematic view of a further embodiment of a reducedpressure transport plant of granular material according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference first to FIGS. 1 to 3, it will be noted that aconventional reduced pressure transport plant of granular materialcomprises a container 1 of any suitable type containing a specificamount of granular material 1 a to be transported, a fluidizing lancemember 2 drawing in granular material 1 a, e.g. formed by asubstantially rigid tube, intended to capture material granules and mixthem with air, as will be further described below. The lance member 2 isin fluid communication with one end of a tube or hose 3, which can be ofboth rigid and flexible type and whose other end penetrates in anintermediate portion of a hermetically-sealed receiver-meter device 4and defines a discharge mouth 3 a.

Within the receiver-meter device 4, at a lower level than that of thedischarge mouth 3 a, a small metering hopper 33 is provided equippedwith lower discharge mouth that can be opened and closed by a bottomflap 34 supported by a projecting arm 15 in turn pivoted at 35 to thehopper, thereby being arranged to oscillate about a horizontal axis. Thedischarge mouth is illustrated closed in FIG. 2, while in FIG. 3 it isopen, in order to discharge granular material 1 a conveyed and collectedin the metering hopper 33 into an underlying hopper 13 set to act as afeed storage of plastic granular material 1 a for a transformer machinegenerally indicated with M.

The rotatable equipment formed by the bottom flap 34 and by the supportarm 15 is provided with a counter-weight 20 that encloses a magnet (notshown in the drawings) and an electromagnetic alignment sensor 21. Withthis structure, when granular material is not present in the meteringhopper 33, the bottom flap 34 comes to automatically close the dischargemouth of the hopper 33, due to the presence of the counterweight 20 andthe vacuum, and the magnet enclosed in the counterweight aligns with themagnetic sensor 21, thus generating an electrical signal. Suchelectrical signal is sent, e.g. by means of electric cable, to areceiving unit, as will be further described below.

The transport air of the granular material 1 a coming from the container1 is separated from the granular material falling inside thereceiver-meter 4 and is suctioned, possibly through a first filter 6,via a mouth 7 a placed in the upper head or portion 5 of thereceiver-meter 4 and in fluid communication with one end of a rigid orflexible duct 7, whose other end leads to a cyclone filtering group 8.The latter is equipped with inner filter 9 with high filtering capacityand traps even small particles dispersed in the air that crosses it.

From the cyclone filtering group 8, a flexible duct 10 departs which isconnected to a vacuum source, typically to the suction mouth of a vacuumpump or a blower 11 provided with an electric control panel 14, thatexpels the air drawn through the ducts 3, 7 and 10 directly into theambient air, e.g. by means of a duct 12.

If the vacuum pump 11 is stopped, the granular material possiblycontained in the meter hopper 33, due to the lack of vacuum and theweight of the granular material therein contained, causes the dischargemouth of the hopper 33 to open, so that any granular material isdischarged into the underlying hopper 13.

When the magnet associated with the counterweight 20 is aligned with themagnetic sensor 21, an electric control signal is generated that is sentto the electric panel 14 of the blower or vacuum pump 11, which is thusactuated, giving rise to a new granular material feed cycle. The cycleis timed and can be varied as a function of the size of thereceiver-meter 4, of its distance from the container 1 and/or of thetype of granular material to be transported.

With a plant of the above-described type, it is possible to carry outthe transport of plastic granular material for distances up to 200 m,even for feeding several machines for the transformation of plasticgranular materials, in which case the plant is called a “centralized”transport plant in jargon. One example of a centralized reduced pressuretransport plant is illustrated in FIG. 4, where a single suction unit(pump or blower) 11 is provided and a cyclone filtering group 8 isarranged upstream of the suction unit. The various receivers-meters 4 ofnumber n, for example 28 receivers-meters, are in fluid communicationwith the filtering group 8 by means of a common duct 70, termed “vacuumline” in jargon. In other words, the vacuum line 70 can serve a number nof transformation machines M1, M2 . . . Mn. Preferably, thereceiver-meters 4 are each equipped with an interception valve (notillustrated in the drawings) placed inside its respective head 5, whichis drivable by a respective electro-pneumatic valve VE1, VE2 . . . ,VEn, in turn, controlled by a suitable electronic control unit ECU setto control every zone of the plant, in particular energizing at one timeone or another receiver-meter 4 according to operating needs. This planttype is particularly indicated for conveying granular material overrelatively large distances, on the order of 200 m. In this case, it isnecessary to employ a very powerful suction unit 11, since the lossloads must be overcome, which are obviously much greater for highdistances, keeping in mind that installing several suction units wouldlead to prohibitive costs.

With every cycle, the conveyance line L1, L2, . . . , Ln is hit with apre-established quantity of air and granular material and at the end ofevery cycle it is completely evacuated of granular material, owing tothe presence of an interceptor device, termed “cleaning valve”, VP1, VP2. . . , VPn provided for each receiver-meter 4, so that when the suctionunit 11 is stopped, the conveyance line L1, L2, . . . , Ln is emptied.One such plant is in particular used when one must feed, in subsequentcycles, different granular materials to several machines for thetransformation of plastic granular material.

Should the conveyance line L1, L2, . . . , Ln be not emptied at thebeginning of every cycle, the tubing could be contaminated or evenobstructed by granules of the previously conveyed material and thesuction unit 11 may not be able to create a sufficient suction effectsuitable for ensuring both the evacuation of the air and the transportof granular material.

One of the problems that occurs in conventional reduced pressuretransport plants of this type is that the flow speed or intensity of thegranules inside the tubes does not remain constant, but varies, up toeven doubling, with the variation of the work conditions.

In FIG. 5, a typical cleaning valve is illustrated, indicated with VP1and inserted in the feed duct 3 of a respective receiver-meter 4. Itcomprises a valve body, in which an air and granular material inletmouth 40 is obtained, where e.g. a nozzle 41 is provided for a firstsection of duct 3 in communication with the respective lance member 2.An outlet mouth is also provided in the valve body, preferably placed inoffset position with respect to the inlet mouth 40, from which a secondsection of the feed duct 3 departs, directed to the receiver-meter 4. Infront of the inlet mouth, but on opposite side thereof, a receivingopening is formed in the valve body for a linear actuator device 43 ofany suitable type, which is set to control a preferably conical plugelement 44, moving it on command of the electronic control unit ECUbetween a closed position, as shown in FIG. 5, in which it closes theinlet mouth 40 or the nozzle 41, and an open position far from the mouth40 or the nozzle 41.

An ambient air inlet opening 45 is also formed in the valve body,externally provided with a filter 46, whereas within the valve body suchopening 45 can be intercepted by the plug element 44 when it is movedinto open position by the actuator 43. With this structure of thecleaning valve VP1, when the plug element is moved into closed positionof the inlet mouth 40 or of the nozzle 41, only ambient air is suctionedthrough the filter 46 and thus through the receiver-meter 4 in order tocarry out a cleaning cycle of the tubing.

In a granular material transport cycle, i.e. when the cleaning valve VP1places the tube section 3 in communication with the lance member 2, withthe second section of tube 3 in communication with its respectivereceiver-meter 4, due to the reduced pressure created by the suctionunit 11, the granular material is caused to move and accelerate until itreaches a so-called “equilibrium” speed.

The initial acceleration imparted to the granular material mainlydepends on the fact that the granular material at the start finds thesecond section of tube 3, that directly communicating with thereceiver-meter 4, to be completely empty, and as it receives granularmaterial, the load losses of the internal air flow and the frictionagainst the walls increase, and consequently the speed of the suctionedair flow decreases. These factors ensure that the acceleration impartedto the plastic granular material 1 a gradually decreases until itreaches the equilibrium speed.

The same occurs when at the cycle's end the linear actuator 43 moves theplug element 44 into closed position against the inlet mouth 40 or thenozzle 41, thus allowing the suction of ambient air through the filter46 in order to start the cleaning of the tubing. In this step, the speedof the plastic material granules present in the second section of theduct 3 tends progressively increase, until complete emptying of thetubing has been obtained, achieving flow intensity values that are evendouble that of the equilibrium speed. At one such speed, the plasticmaterial granules 1 a scrape against the walls of the tubes, inparticular at the curved sections of the tubes; consequently, a thinfilm is deposited, especially at rough areas of the material (usuallymetal) composing the tube, giving rise to the angel-hair phenomenonmentioned above.

With reference to the embodiment of the present invention illustrated inFIG. 6, a reduced pressure transport plant of granular materialscomprises one or more granular material containers or silos 100, fromwhich such material is suctioned by means of one or more suction units11, e.g. formed by one or more vacuum pumps, and a gaseous medium orfluid, e.g. air or nitrogen, which brings the granular material 1 atherewith.

The various containers 100 of granular material 1 a are in fluidcommunication by means of a respective duct L1, L2, . . . , Ln with arespective receiver-meter RD1, RD2, . . . , RDn, each duct L1, . . . Lnbeing interceptable by a respective cleaning valve VP1, VP2, . . . ,VPn.

The outlet for air from each receiver-meter RD1, RD2, . . . , RDn isconnected to a common vacuum line LV, in which an air flow rate meter MPis provided, e.g. comprising a Venturi meter of any suitable type, whichis electrically connected with an electronic control unit ECU.

Moreover, the plant comprises one variator device DV per suction unit11, which is arranged to vary the power or typically the rotation speedof the electric motor (not shown in the drawings) for actuating therespective suction units. Such speed variator device is preferably ofelectronic type, e.g. a so-called inverter, of any suitable type, whichis intended to vary the frequency of the power supply current to themotor of its respective suction unit, and is in turn controllable by theelectronic control unit ECU.

The air flow rate meter MP is designed to send electrical signals to theinput of the electronic control unit ECU which are correlated to the airflow rate in the vacuum line LV. The electronic control unit ECUprocesses the signals received in input in order to generate controlsignals to be sent to the speed variator device(s) (inverter(s)) DV,which correspondingly vary the frequency of the power supply current tothe motor of the suction unit(s) 11, 11 a. In this manner, thedepressurization or vacuum level and consequently the speed of thegranular material 1 a traveling along the tubes is adjusted as afunction of the variations in the transport conditions of the material,which as stated above can vary when passing, for example, from thefilling step to the unloading step of the granular material in thevarious suction lines L1, L2, . . . , Ln of the granular material 1 a.

More particularly, since there is a correlation between the parametersformed by flow rate, air speed and the vacuum level inside the tubes,the electronic control unit ECU through the inverter(s) DV modulates therotation speed of the motor, and thus the power of each suction unit 11,thereby producing an initial acceleration ramp of the granular material1 a as a function of the variation of the depressurization or vacuumlevel. Subsequently, when an increase occurs of the load lossesfollowing the deposit of granular material on the inner surface of thevacuum line LV, the flow rate meter MP detects the flow rate variationcaused by the load losses, which results in the variator device(s) DV(inverter(s)) increasing its rotation speed and thus the power of therespective suction unit 11, 11 a. In such a manner, the flow ratedecrease is gradually compensated, thus maintaining the granularmaterial 1 a movement speed constant over time along the ducts, or thusobtaining, if the circumstances require it, a variable speed progressionover time.

In the tube cleaning step, on the other hand, the reverse processoccurs. Once the feed of granular material 1 a to the respectivereceiver-meter RD1, RD2, . . . , RDn is stopped, the air speed in thereduced pressure tubing increases. The flow rate meter MP consequentlydetects a flow rate variation and sends a corresponding signal to theelectronic control unit ECU, which will consequently drive the speedvariator device(s) DV.

A control microprocessor (not shown), e.g. a PLC of any suitable typeplaced in the electronic control unit ECU, is set to create differenttransport condition profiles as a function of the type of granularmaterial 1 a to be transported. Typically, in a first storage portion ofthe control microprocessor, a table is pre-stored, which is none otherthan a list of a first array of plastic granular materials 1 a withtheir respective characteristic parameters of their respective optimaltransport speed profile. In a second storage portion, the operator ofthe reduced pressure transport plant can store the parameters ofpossible new granular materials, defined “experimental”, through asuitable user interface, e.g. consisting of a video unit (monitor) andaccess means to the microprocessor for the data insertion, e.g. akeyboard and/or a mouse. Preferably, the user interface is a graphicalinterface with objects of “touch-screen” type.

With such device, it is possible to process any granular material 1 a,feeding it at the most suitable speed, without generating powders,eliminating possible speed peaks, reducing the transport tube wear bythe conveyed granular materials, optimizing the various cycles in acompletely automatic manner, without risking the obstruction of thetransport ducts, adapting the performances and productivity of the plantas a function of the transported granular material and eliminating everyimpact of the filtering effect on the speed and/or reduced pressurelevel prevailing during the transport.

According to an advantageous variation of a reduced pressure pneumatictransport plant according to the present invention, the suction units 11and 11 a or possibly further provided suction units, all equipped with arespective inverter DV, operate for example in stand-by since they areconnected in parallel with each other, and are intended to beginoperating in an alternating manner or simultaneously if conditionsrequire it in order to increase the power, i.e. the depressurizationlevel in the vacuum line LV and in the receiver-meters RD1, RD2, . . . ,RDn.

A reduced pressure transport plant as described above can be used withonly one suction unit 11 in order to ensure the feed of granularmaterial 1 a to a single transformation machine or array oftransformation machines M1, M2, . . . , Mn.

Another reduced pressure transport plant of granular material accordingto the present invention will be described below with reference to FIG.7, where the same reference numbers are used for indicating componentsalready described with reference to the embodiment of FIG. 6. Such plantprovides for the presence of a suction unit 11 equipped with inverterDV. Advantageously, one or more auxiliary suction units 11 a can beconnected in parallel to the suction unit 11, also equipped withinverter DV, similar to that described with reference to the embodimentillustrated in FIG. 6.

Also provided is a reduced pressure storage tank SER of any suitabletype, into which the various vacuum lines LV1, LV2, . . . , LVn convergeof the respective receivers-meters RD1, RD2, . . . , RDn serving arespective transformation machine M1, M2, . . . , Mn. The tank SER isarranged upstream of the suction unit 11.

Preferably, downstream of the tank SER, a filtering group F is providedfor, to which the air suctioned by the tank SER is directed in order tobe filtered before reaching the suction unit(s) 11, 11 a. Moreover,according to such embodiment a differential pressure meter DPS is alsoprovided for of any suitable type, intended to measure the load loss dueto the obstruction of the filtering group F and to generate respectiveelectrical signals to be sent to the input of an electronic control unitECU.

Starting from each container 100 of granular material to be transferred,a feed duct L1, L2, . . . , Ln departs that is intended to feed granularmaterial to a respective receiver-meter RD1, RD2, . . . , RDn. Providedin series in each conveyance line L1, L2, . . . , Ln is both a cleaningline VP1, VP2, . . . , VPn and a detection means RS1, RS2, . . . , RSnof the speed of the granular material that moves inside the respectivefeed line, e.g. comprising a sensor known in the state of the art andbased on the interaction of the flow of the solid material moving in thefeed line with a suitable electromagnetic signal, e.g. low energymicrowaves, which send corresponding control signals to the input of theelectronic control unit ECU.

In each vacuum line LV1, LV2, . . . , LVn, the following are provided inseries:

-   -   an air flow rate meter VT1, VT2, . . . , VTn, e.g. comprising a        Venturi meter of any suitable type,    -   a pressure meter PS1, PS2, . . . , PSn intended to measure the        pressure in the respective vacuum line LV1, LV2, . . . , LVn and        to send corresponding signals to the input of the electronic        control unit ECU, and    -   a motorized valve MV1, MV2, . . . , MVn of any suitable type        arranged to maintain a correct reduced pressure or vacuum level        in the respective vacuum line LV1, LV2, . . . , LVn as well as        in its respective receiver-meter RD1, RD2, . . . , RDn.

The electronic control unit ECU is designed to process the signalsreceived in its input and to send, if deemed necessary, control signalsto one or more of the motorized valves MV1, MV2, . . . , MVn, therebyobtaining a desired speed profile for each specific granular material tobe fed to the transformation machines M1, M2, . . . , Mn, as well as tothe speed variator device(s) DV, which modulate the rotation speed andthus the power of the respective suction units 11, 11 a, thus alwaysmaintaining a desired reduced pressure level or vacuum level in thevacuum storage tank SER.

The electronic control unit ECU is suitable for diversifying thefunctioning parameters in the various vacuum lines LV1, LV2, . . . , LVnand in its respective receiver-meters RD1, RD2, . . . , RDn based on thepre-established movement speed for every type of granular materialinside each conveyance line L1, L2, . . . , Ln.

Of course, also in this embodiment, through a control microprocessor ofthe electronic control unit ECU, e.g. a PLC, of any suitable type, it ispossible to store different transport condition profiles as a functionof the material type for every transport line.

Alternatively, if it is desired to diminish the plant costs, in place ofthe pressure meters PS1, PS2, . . . , PSn a single pressure meter can beprovided, such meter being designed to carry out the measurement of thereduced pressure in the vacuum storage tank SER and to sendcorresponding control signals to the input of the electronic controlunit ECU.

The above-described plant is susceptible to numerous modifications andvariations within the protection scope as defined by the claims.

Thus, in place of suction means of the air or another gaseous fluid,pressing or pressurizing means can be provided, obtaining entirelysimilar results.

What is claimed is:
 1. A plant for the transport of plastic granularmaterial, comprising: at least one container for plastic granularmaterial to be transported, at least one or more receiver-meters one ormore conveyance ducts each of which being in fluid connection with saidat least one container and said at least one or more receiver meters,depressurization structure for creating a relative vacuum, at least onevacuum duct in fluid connection with said depressurization structure andsaid at least one or more receiver-meters, each of said one or moreconveyance ducts being in fluid connection with said at least one vacuumduct through said at least one or more receiver-meters, a speeddetection structure located in each of said vacuum ducts, an electronicspeed detection control with structure in communication with said speeddetection structure, said depressurization structure, and said at leastone or more receiver-meters, and a cleaning valve in fluid connectionwith each of said conveyance ducts, wherein a flow of plastic granularmaterial from each of said containers can be individually andautomatically adjusted.
 2. A plant according to claim 1, wherein saiddepressurization structure comprises at least one rotatable componentactuated by an electric motor and a power adjusting structure having atleast one rotation speed variation device of at least one rotatingcomponent of said depressurization structure.
 3. A plant according toclaim 2, wherein said power adjusting structure comprises an inverter tovary the frequency of power supply current to said electric motor insaid depressurization structure.
 4. A plant according to claim 1,wherein said depressurization structure comprises at least one vacuumpump.
 5. A plant according to claim 1, wherein said speed detectionstructure comprises measuring structure of air speed.
 6. A plantaccording to claim 5, wherein said speed detection structure comprisesat least one flow rate meter.
 7. A plant according to claim 1,comprising at least one reduced pressure storage tank arranged upstreamof at least one depressurization structure and in fluid communicationwith at least one vacuum duct of at least one receiver-meter.
 8. A plantaccording to claim 7, comprising, downstream of said at least one tank,at least one filtering structure through which fluid suctioned from saidat least one tank is caused to flow.
 9. A plant according to claim 8,comprising: a differential pressure meter structure arranged to measurepossible load losses due to obstruction of said at least one filteringstructure and to generate respective electrical signals to be sent toelectronic control unit.
 10. A plant according to claim 9, comprising: ameter structure of the fluid flow rate, a pressure meter structure tomeasure the pressure in a respective vacuum duct and to send respectivesignals to the input of said electronic control unit structure, andvalve structure arranged to maintain a correct reduced pressure orvacuum level in a respective vacuum line as well as in a respectivereceiver-meter.
 11. A plant according to claim 1, comprising in eachconveyance line, a respective detection structure of the speed of thegranular material which moves inside its respective conveyance duct,said detection structure being intended to send a signal to saidelectronic control unit structure.
 12. A plant according to claim 1,comprising a user interface video unit and data insertion structure,said user interface being intended to store, in said electronic controlunit structure, treatment parameters and characteristics related to thegranular material(s) to be treated.
 13. A plant according to claim 1,wherein said electronic control unit structure comprises a storageportion designed to store characteristic data of the transport speedprogression of specific granular materials.